Abstract

The scattering parameters of an amplifier based on a dc superconducting quantum interference device are directly measured at 4.2 K as functions of the bias current and applied magnetic flux. These parameters are used to determine the stability of the amplifier with arbitrary source and output load impedances. It was found that the amplifier is conditionally stable, and that the stability is improved by decreasing the gain or adding negative feedback. With suitable bias selection, the amplifier is shown to be sufficiently stable to allow operation with a resonant source impedance.

Received 18 January 2010Accepted 10 March 2010Published online 26 April 2010

Acknowledgments:

A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Intelligence Advanced Research Projects Activity (D.K.). This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (J.C.).